Recycling of paper from a wide variety of sources is quickly becoming the norm. However re-use is often difficult and technology is often lacking for economical ways to re-use discards. As we re-use more and more, we continually need new ways to recycle lower and lower grades and incorporate the result in higher and higher grade products. In the case of paper, it is relatively easy to recycle the fiber in office waste but curbside papers are an entirely different matter.
Most office waste is primarily Kraft fiber contaminated with black xerographic ink so efficient techniques for recycling this grade are becoming increasingly common. Presently, almost 100 percent of the highest quality grades of paper, like printer's clippings and white office waste, is recycled. Those grades are considered premium secondary or recycled fiber sources as the original, high quality of the chemically produced virgin paper fibers makes it relatively inexpensive to recycle those fiber sources into a high brightness pulp. Accordingly, this class of waste-paper, and therefore the pulp therefrom, commands premium pricing.
However, curbside waste paper also comprises large amount of mechanical pulp contaminated with a variety of inks along with numerous other unmentionables. Recycling of papers, like those from curbside residential recycling, is more costly and difficult. Because those papers tend to include substantial groundwood content, the current technology used to recycle premium fibers is ill-suited for these lower grades of waste paper. Accordingly, recycling of mixed waste fiber streams presents a far more severe challenge as techniques which are suitable for bleaching of Kraft fiber may not be as well-suited for brightening of mechanical pulps. Particular problems with recycling mixed waste streams relate to fines, trash, scale, yield, and, of course, cost. Many of the techniques known for recycling of streams primarily comprised of Kraft fiber suffer from deficiencies which become aggravated when lower grade fibers are included, particularly when it is desired to use the resulting fiber for manufacturing absorbent papers. Accordingly, curbside waste paper tends to be far more modestly priced than office waste.
More specifically, the lower grade pulp fibers used in the original production of those curbside waste paper products suffer from a decreased brightness over the premium virgin or premium recycled fibers. Thus, the recycled fibers from those curbside waste paper products cannot easily be used to create premium or near-premium quality consumer products like bath tissue, facial tissue, paper towels, and napkins, since consumers tend to prefer higher brightness fibers in these products. Such products are considered premium or near premium due to, for example, their high brightness and/or low ink concentration. Even though current technologies make it possible to recycle some of those curbside waste papers, the high cost makes their use unattractive. In fact, in certain instances, the lower purchase cost of those curbside waste papers is often considerably or even completely offset by the higher cost of chemical treatments needed to produce acceptable brightness levels.
United States Patent Application Publication No. 2008/0087390 to Lee et al. relates to a method of bleaching recycled fibers comprising contacting the recycled fibers with at least an activating bleach step, an alkaline hydroxide step, and a reductive bleach step, wherein the activating bleach step comprises contacting the recycled fibers with at least one activating bleaching agent chosen from peracetic acid, peroxymonosulfuric acid, and an acylamide such as TAED.
U.S. Pat. No. 7,384,502 to Kamijo et al. relates to a process for preparing bleached mechanical pulp from wood chips by impregnating with a chemical liquor consisting essentially of an aqueous solution of an alkaline inorganic compound and a chelating agent at a pH range of 7-12. Specific examples of such impregnating agents include, e.g. aqueous solutions of alkaline inorganic compounds such as sodium hydroxide and potassium hydroxide, preferably aqueous sodium hydroxide solutions. Suggested chelating agents include diethylenetriaminepentaacetic acid, 2-hydroxyethylethylenediaminetriacetic acid, ethylenediaminetetraacetic acid, diethylenetriaminepenta(methylenephosphonic) acetic acid, or alkaline metal salts thereof. The liberated pulp is then bleached with oxidizing agents such as hydrogen peroxide, ozone and peracetic acid or with reducing agents such as sodium hydrosulfite (sodium dithionite), sodium hydrogen sulfate, sodium borohydride and formamidine sulfinic acid (FAS). Only the use of sodium hydroxide is exemplified in connection with their peroxide bleaching. It is claimed that mechanical pulps having high brightness can be prepared from even wood species previously considered to be unsuitable for mechanical pulps such as materials having low bleachability containing high levels of extractives. The process is said to expand the application of wood species that were difficult to convert into mechanical pulp, thus greatly contributing to environmental protection in terms of more effective use of wood. Moreover, the amount of bleaching agents used can be reduced.
U.S. Pat. No. 7,297,225 to Thomas et al. is one of the few references dealing directly with the issues involved in brightening of recycle fiber stating that:                Typically the pulp for use in the present invention has previously undergone deinking and pulping . . . the bleaching process of the present invention may be incorporated at any point in the pulping process, according to one embodiment, the bleaching is carried out immediately after deinking of the pulp. Removal of contaminants that interfere with the bleaching process result in higher bleaching efficiencies. Compositions which may be introduced to the pulp for bleaching include hydrogen peroxide, and may include other bleaching agents including but not limited to, one or more of alkali hydroxide, gaseous oxygen, ozone, and peroxygen compounds (including, but not limited to, peracetic and peroxymonosulfuric acid). The bleaching agents may further include reductive agents (including, but not limited to, formadmidine [sic, formamidine]sulfinic acid (FAS), hydroxymethane sulfinic acid (HAS), sodium borohydride, and sodium hydrosulfite), and mixtures thereof. In one embodiment according to the present invention, hydrogen peroxide, sodium hydroxide, and gaseous oxygen are all added to the pulp for bleaching. Optionally, catalyzing or activating agents may be added.        
Thomas points out that “bleaching with hydrogen peroxide has its drawbacks, because the bleaching effect is not as strong as with chlorine-based bleaches” and that:                Simply using greater amounts of hydrogen peroxide in the bleaching process does not solve the problem since merely increasing the amount of hydrogen peroxide results in large amounts of hydrogen peroxide remaining unreacted and therefore wasted.        
Thomas discounts the concept of using multi-stage peroxide bleaching systems as “necessarily more costly and more complex to operate than single-stage bleaching systems” and instead bleaches at high temperature and pressure, while preventing flashing upon discharge by:                . . . use of a heat exchanger and a cooled recycle filtrate stream from the bleaching process . . . [to] . . . increase one or more of the efficiency, effectiveness, and safety of high temperature peroxide bleaching procedures.        
While mentioning the use of alkali hydroxide in association with hydrogen peroxide, gaseous oxygen and peroxygen compounds, at an initial pH in the range of 10 to 11 or, in a preferred embodiment 10.4 to 10.6, the only specific alkali hydroxide mentioned by Thomas is sodium hydroxide.
U.S. Pat. No. 7,163,564 to Sibiet et al. relates to a method for brightening virgin mechanical pulp using a combination of (i) an aqueous solution comprising sodium borohydride and sodium hydroxide; and, (ii) an aqueous solution comprising sodium bisulfate.
U.S. Pat. No. 7,052,578 to Wajer et al. relates to bleaching of wood pulp:                using hydrogen peroxide as the oxidative bleaching agent in the presence of magnesium hydroxide or magnesium oxide. The bleaching process is carried out in the presence of magnesium hydroxide as the predominant, and preferably essential, source of alkali. The process optionally includes transition metal chelants, such as DTPA or EDTA in the bleaching slurry. The process eliminates the need for added caustic and silicate in such systems and can be carried out at or near neutral pH of 5.0 to 8.5.        
Wajer et al. present numerous and extensive examples in which the amounts of Mg(OH)2, H2O2, chelants, and locations for additions of various ingredients are investigated. Without equaling the brightness results obtained using sodium hydroxide and hydrogen peroxide and concluded that the technology:                . . . reduces chemicals costs by eliminating caustic soda and sodium silicate, and by reducing DTPA and hydrogen peroxide usage.        . . . eliminates scaling and abrasion caused by sodium silicate. Allows bleaching to occur in the refiners.        . . . provides comparable brightness to caustic soda and sodium silicate bleaching at a significantly lower pH.        . . . provides for peroxide bleaching at a lower pH, which potentially reduces pH adjustment costs downstream.        . . . improves bulk properties of bleached pulp compared to caustic soda.        . . . provides a divalent magnesium, which improves the dewatering properties of pulp thus reducing the need for chemicals and defoamers. The divalent magnesium ion can also aid in better settling for wastewater treatment operations.        . . . reduces organics (BOD/COD) in the bleaching effluent for lower wastewater treatment costs.        . . . provides for recycling of high peroxide residuals for a reduction in peroxide usage.        . . . provides for improved pulp strength properties compared to caustic soda.        and        . . . provides reduced anionic trash and cationic demand for improved papermaking operations.        
U.S. Pat. No. 7,001,484 to Ni et al. relates to a process for peroxide bleaching of mechanical or high yield pulp, the process comprising adding to a pulp slurry at least one stabilizer for stabilizing low valency states of transition metal ions in the slurry; subsequently adding caustic soda to said slurry simultaneously with or subsequent to adding said at least one stabilizer; adding hydrogen peroxide to the slurry at a preselected point; and subjecting the slurry to preselected conditions to complete the bleaching process.
U.S. Pat. No. 6,866,749 to Delmas et al. relates to a method for bleaching different types of paper pulp in two steps at atmospheric pressure and at a temperature not higher than 100° C. The first step consists in contacting the unbleached paste [sic, pulp?] with a mixture of peracetic acid and performic acid. The second step consists in treating the bleached pulp derived from the first step, with a solution of soda and hydrogen peroxide. The resulting pulps are said to exhibit a high index of whiteness and a degree of polymerisation close to unbleached pulps.
U.S. Pat. No. 6,881,299 to Parrish et al. relates to methods of bleaching mechanical pulp under alkaline conditions with hydrogen peroxide. The methods include introducing a source of magnesium, hydroxyl and perhydroxyl ions to a refiner, typically in the form of magnesium oxide, water and hydrogen peroxide. Parrish teaches that:                Peroxide bleaching with sodium hydroxide/sodium silicate chemicals generates calcium oxalate scale . . . [resulting in] . . . tenacious deposits on the equipment . . . [that] can end up in the finished paper product and cause problems with the paper press [while] Magnesium ions, on the other hand, react with oxalate ions to form magnesium oxalate that is more soluble than calcium oxalate, thus reducing scale . . . .        Magnesium oxide/hydroxide and hydrogen peroxide bleaching has the advantage of eliminating the use of sodium silicate. The high anionic charge associated with sodium silicate interferes with downstream paper machine retention aid chemistry. Silicates along with other process materials contribute to the conductivity and negative charge of the water. The elimination of sodium silicate should result in improved paper machine retentions, and allow for retention aid optimization . . . .        Using a magnesium oxide and water slurry as the substitute for sodium hydroxide and sodium silicate in a refiner lowers bleaching times . . . reduces cost . . . can be applied to any refiner bleaching process . . . [and] can be used for high consistency mechanical pulps, as well as recycled pulps from post consumer sources, and chemical pulps, such as Kraft and sulfite pulps that are processed through a refiner . . . .        The brightness achieved by hydrogen peroxide bleaching using magnesium oxide/hydroxide/water is comparable to using sodium hydroxide/sodium silicate without the drawbacks of sodium hydroxide/sodium silicate and with no impact on bleaching efficiency.        
U.S. Pat. No. 6,569,286 to Withenshaw et al., discusses the formation of a solution of peracetic acid by reacting solid (N,N,N′,N′-tetraacetylethylenediamine, TAED) granules in water in the presence of hydrogen peroxide, a chelant, and caustic soda. The solution may be used to bleach a pulp in a single stage under alkaline conditions. Withenshaw et al., also discusses that the use of its pre-reacted TAED solution is superior to use of TAED either directly added to the pulp mixture or pre-reacted with any other type of peroxide solution. The single stage process is also supposedly superior to a process using an alkaline peroxide bleaching stage.
U.S. Pat. No. 6,743,332 to Haynes et al. relates to alkali peroxide bleaching of thermomechanical pulps at high temperatures, using an alkali buffer (such as soda ash or magnesium hydroxide), instead of sodium hydroxide. Buffering the system at lower pH (about 9 to about 10.5) is said to prevent peroxide decomposition and alkali darkening, but still provide adequate alkali to produce effective bleaching. The buffer is said to release alkalinity as necessary providing just enough alkalinity for slow, even production of perhydroxyl ions as needed for bleaching, prolonging the effective bleaching time, making the peroxide more effective and giving higher brightness and higher yields by reducing the breakdown of the wood fibers.
U.S. Pat. No. 6,632,328 to Wan et al. relates to a method for bleaching unbleached softwood or hardwood pulps using hydrogen peroxide without added alkali using an alkaline earth metal carbonate, preferably magnesium carbonate at elevated temperatures.
U.S. Pat. No. 6,524,437 to Vincent et al. relates to a process for peroxide bleaching of pulp using magnesium oxide or magnesium hydroxide as a substitute for caustic soda, the pulp being bleached in the presence of hydrogen peroxide characterized in that the concentrations of iron and manganese is controlled using DTPA, DTMPA, EDTA and/or HEDTA as chelants.
Several patents discuss using solid TAED as a component of a detergent composition, which are intended to be used to clean textiles and clothing. The issue addressed in these patents is achieving long term stability of the composition and activation of the bleach for final use. For example, U.S. Pat. No. 6,225,276 to Gassenmeier et al. coats the TAED compound with a polymeric acid, which then dissolves when the detergent composition is added to water to allow release of bleaching compounds. In addition, U.S. Pat. No. 4,283,302 to Foret et al., U.S. Pat. No. 4,338,210 to Clements et al., U.S. Pat. No. 4,938,889 to Wilsberg et al., U.S. Pat. No. 6,080,710 to Withenshaw et al., and U.S. Pat. No. 5,478,357 to Madison et al., all discuss the need to produce dry, shelf-stable detergent compositions that generate active bleaching compounds when wetted.
U.S. Pat. No. 6,221,209 to Desprez et al. relates to a process for bleaching chemical paper pulp to a brightness of at least 89° ISO, in a treatment sequence of several stages including a final bleaching stage with hydrogen peroxide in alkaline medium carried out in the presence of at least one stabilising agent and at a consistency of at least 25% by weight of solids.
U.S. Pat. No. 5,733,412 to Markham et al. discusses a method to decolorize unbleached Kraft “brown” fibers that may be a contaminant in mixed wastepapers. Markham's method requires fine screening mixed wastepaper, followed by a two-step bleaching process. That two-step process must alternate oxidative and reductive bleaching, where if the first step is oxidative then the second step must be reductive, and alternately if the first step is reductive then the second step must be oxidative. The first bleach step is carried out in a dispersion machine and at least one step must be carried out at high temperature (above 100° C.) and high pressure (exceeding one atmosphere). While Markham et al. disclose several oxidative and reductive agents, in its bleaching steps only a first FAS (thioureas dioxide) step followed by a second hydrogen peroxide step is preferred and practiced in the examples.
U.S. Pat. No. 5,645,686 to Troughton et al. discusses a three- to six-step bleaching process for chemical pulps, in which at least one of the steps involves contacting the pulp with an enzyme. Troughton et al. do not discuss the use of a reducing agent.
U.S. Pat. No. 5,589,032 to Chang et al. discusses a process to increase the generation efficiency of peracids from hydrogen peroxide. That method adds peroxymonosulfuric acid (Caro's acid) to a reaction vessel containing concentrated hydrogen peroxide and acetic acid. The addition of Caro's acid supposedly increases the generation of peracids. U.S. Pat. No. 5,693,185 also to Chang et al. discusses the use of a mixed peracid solution to brighten lignocellulosic and cellulosic pulps, for example, delignified wood pulps and cotton or cotton by-products.
U.S. Pat. No. 5,387,317 to Parthasarathy et al. discusses a method to delignify chemical pulp “brown stock” using a high temperature, high pressure process combining peracetic acid, ozone, and oxygen under acetic conditions.
U.S. Pat. No. 5,296,100 to Devic relates to a process for bleaching high-yield lignocellulosic wood pulps by (i) first pretreating such pulp with a complexing agent for metal ions and next washing the pretreated pulp, and then (ii) bleaching such pretreated/washed pulp with an initial amount of hydrogen peroxide in an alkaline medium, including adding a supplementary bleaching amount of hydrogen peroxide and a supplementary amount of an alkaline agent to the pulp over the course of the bleaching step (ii) without interrupting same, at a point in time when from 60% to 85% of the initial amount of hydrogen peroxide has been consumed, and such supplementary amount of hydrogen peroxide being equal to or less than the initial amount thereof.
U.S. Pat. No. 5,264,001 to Arifoglu et al. relates to a process for sequential oxidative and reductive bleaching of fibers in a single bath, which provide superior bleaching with less physical damage. Said processes comprising the steps of: (1) bleaching fibers with hydrogen peroxide; (2) adding either, (a) a material which combines with hydrogen peroxide to form a reductive bleaching agent, or (b) an inactivating material to inactivate unspent hydrogen peroxide with subsequent addition of a reductive bleaching agent; (3) reductively bleaching the already oxidatively bleached fibers; and, (4) adding an oxidizing material in an amount at least sufficient to oxide excess reductive bleaching agent.
U.S. Pat. No. 5,223,091 to Hetzler et al. relates to a process for brightening mechanical pulps by chelating to control the manganese content of the pulp to less than 30 parts per million and copper content less than 1 part per million by adding ions preferably magnesium ions as magnesium sulphate MgS04 in the amount of 400 to 3,000 ppm Mg ions retained by the pulp based on the oven dry weight of the pulp and thereafter applying a bleaching liquor composed of peroxide as hydrogen peroxide and an alkali as sodium hydroxide and containing no added sodium silicate.
U.S. Pat. No. 4,756,798 to Lachenal et al. relates to a process of bleaching mechanical pulp with hydrogen peroxide wherein the mechanical pulp is subjected to oxygen pressure prior to or simultaneously with said peroxide treatment.
U.S. Pat. No. 4,548,674 to Hageman discusses the problem of removing polymeric tape contaminants from wastepaper during a recycling process. Peracetic acid is applied to contaminated wastepaper at an acid pH (between 2.5 to 6.5) and aids in the breakdown of the adhesive polymers. No brightness gain is achieved through the peracetic acid treatment at the disclosed operating conditions.
U.S. Pat. No. 4,400,237 to Kruger et al. discusses a process for bleaching cellulose using a two-step process, wherein organic peracids are applied to the pulp at an acid pH followed by a hydrogen peroxide step at alkaline pH.
A three-stage oxidative bleaching stage for bleaching chemical lignocellulosic pulps is discussed in U.S. Pat. No. 4,372,812 to Phillips et al. That process uses an oxygen bleaching stage followed by a peroxide bleaching stage followed by at least one ozone bleaching stage. The peroxide bleaching stage may use alkaline hydrogen peroxide, acid hydrogen peroxide, or a peracid bleaching agent. The patent discusses that each bleaching stage should be followed by a washing stage to remove residual chemicals and bleaching byproducts, and also discusses the wash filtrates may be utilized in a countercurrent flow where the filtrate from the following stage is utilized as the wash water in the preceding stage.
U.S. Pat. No. 4,187,141 to Ahrel relates to a process of producing mechanical pulp in a defibration apparatus in which wood chips are ground between a pair of discs in a pressurized grinding zone. Prior to defibrating, the chips are impregnated with a solution of alkali, selected from the group consisting of sodium hydroxide, alkali silicate, alkali carbonate and alkali bicarbonate, and peroxide, surplus impregnating solution is removed from the chips by compression of the chips, the chips are introduced into a pressure vessel in communication with the grinding zone and compressed air is introduced into the pressure vessel in an amount sufficient to maintain the chips in the pressure vessel at a temperature below 90° C. and to maintain superatmospheric pressure within the defibrating zone.
The use of peracetic acid has been suggested as a component of elemental chlorine-free bleaching and/or delignification sequences for chemical pulps. U.S. Pat. No. 3,720,577 to Roymoulik discusses a two-stage process that may involve a chlorine dioxide bleaching step followed by washing and a peracetic acid bleaching step. A three-stage process is also discussed using chlorine dioxide followed by peracetic acid followed by an additional chlorine dioxide bleaching step. Pulp washing is practiced after each bleaching step. In U.S. Pat. No. 3,695,995, Roymoulik further discusses a two- or three-stage elemental chlorine free process for bleaching chemical pulps that utilizes an oxygen stage as the first bleaching step, which must have a “protector” compound added to prevent degradation of the cellulose fibers. The protector is a polysulfide having the formula Na2Sx, where x is an integer from 1 to 4, and/or Na2S2O4.
U.S. Pat. No. 3,867,246 to Hebbel et al. relates to bleaching of cellulose in an aqueous medium in several steps using an inorganic or organic peroxide or hydroperoxide under alkaline conditions, an organic percarboxylic acid or water soluble salt thereof and an inorganic or organic peroxide or hydroperoxide under alkaline conditions.
European Patent No. 0148712 to Dubreux relates to a process for bleaching chemical pulps using hydrogen peroxide, in a single stage at a pH of 11 to 11.5, at a temperature of 70° C. to 100° C. in a solution containing hydrogen peroxide, at least one alkaline agent chosen from sodium hydroxide and sodium carbonate, at least one alkali metal silicate, at least one magnesium salt and at least one calcium salt these elements being maintained in the dissolved state with a complexing agent for alkaline-earth ions.
International Patent Application Publication No. WO 96/41917 (also U.S. Pat. No. 6,056,853 to Vincent et al.) relates to a process for peroxide bleaching of pulp using magnesium oxide as the sole alkaline source wherein said pulp is bleached in the presence of hydrogen peroxide for a maximum period of 180 minutes and achievement of a target ISO brightness of 65 in regard to freshly prepared pulp characterised in that magnesium oxide is utilised as MgO particles having a particle size of 5-500 microns and a particle surface area (PSA) of between 20-60 m2/g.